Two-stage cascade refrigeration systems are known for cooling spaces such as the interior of cabinets, for example, to temperatures well below zero degrees centigrade, such as temperatures below −40° C., for example. For example, freezers of the type known as ultra-low temperature freezers (“ULT's”) are known to use this type of refrigeration system and are used to cool cabinet interiors to temperatures as low as about −80° C. or even lower.
Refrigeration systems of this type are known to include two stages circulating respective first and second refrigerants. The first stage transfers energy (i.e., heat) from the first refrigerant to the surrounding environment through a condenser, while the second refrigerant of the second stage receives energy from the cooled space (e.g., a cabinet interior) through an evaporator. Heat is transferred from the second refrigerant to the first refrigerant through a heat exchanger that is in fluid communication with the two stages of the refrigeration system.
Conventional two-cascade refrigeration systems utilize compressors each having a single, fixed speed, and conventionally having the same maximum capacity. In this regard, operation of the system may entail simply activating and deactivating each of the two compressors at various times. The ability of systems of this type to attain a uniform temperature in the cooled space, however, is limited, and the efficiency of operating such systems is also limited, as is the life expectancy of the systems themselves. In addition, operating one or both of the compressors at maximum capacity may be detrimental, while operating one or both of the compressors at a capacity lower than the maximum capacity for that compressor results in operational inefficiencies. Further, conventional two-cascade refrigeration systems are known to operate at a single predetermined level of noise during steady-state operation.
It would be desirable, therefore, to provide a refrigeration system that addresses these and other problems associated with conventional two-stage cascade refrigeration systems.